Magnetic and Luminescence Properties of 8-Coordinate Holmium(III) Complexes Containing 4,4,4-Trifluoro-1-Phenyl- and 1-(Naphthalen-2-yl)-1,3-Butanedionates

A new series of mononuclear Ho3+ complexes derived from the β-diketonate anions: 4,4,4-trifluoro-1-phenyl-1,3-butanedioneate (btfa−) and 4,4,4-trifuoro-1-(naphthalen-2-yl)-1,3-butanedionate (ntfa−) have been synthesized, [Ho(btfa)3(H2O)2] (1a), [Ho(ntfa)3(MeOH)2] (1b), (1), [Ho(btfa)3(phen)] (2), [Ho(btfa)3(bipy)] (3), [Ho(btfa)3(di-tbubipy)] (4), [Ho(ntfa)3(Me2bipy)] (5), and [Ho(ntfa)3(bipy)] (6), where phen is 1,10-phenantroline, bipy is 2,2′-bipyridyl, di-tbubipy is 4,4′-di-tert-butyl-2,2′-bipyridyl, and Me2bipy is 4,4′-dimethyl-2,2′-bipyridyl. These compounds have been characterized by elemental microanalysis and infrared spectroscopy as well as single-crystal X-ray difraction for 2–6. The central Ho3+ ions in these compounds display coordination number 8. The luminescence-emission properties of the pyridyl adducts 2–6 display a strong characteristic band in the visible region at 661 nm and a series of bands in the NIR region (excitation wavelengths (λex) of 367 nm for 2–4 and 380 nm for 5 and 6). The magnetic properties of the complexes revealed magnetically uncoupled Ho3+ compounds with no field-induced, single-molecule magnet (SMMs).


Introduction
The luminescent emissions of lanthanides in general, and specifically holmium complexes, have been known for decades, as they play crucial roles in research and have a wide range of useful applications . Compared to other lanthanides, holmium was proved to serve as a good candidate to make quantum computers, where one bit of data can be stored on a single holmium atom set on a bed of magnesium oxide [23,24]. In addition, Ho is used to generate the strongest artificial magnetic fields when placed within high-strength magnets [25]; Ho-dopped yttrium iron garnet is used in optical insulators, microwave equipment, and in solid-state lasers [26], and is one of the colorant's sources for yellow and red colors in glass and cubic zirconia [27].
The magnetic susceptibility and magnetization measurements were performed with a Quantum Design MPMS-XL SQUID magnetometer at the Magnetic Measurements Unit of the University of Barcelona. Pascal's constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the corrected molar magnetic susceptibilities.

Synthesis of the Complexes
To a methanol solution (10 mL) containing NaOH (6 mmol, 0.240 g), Hbtfa was added in an amount of 6 mmol, 0.130 g, and HoCl 3 ·6H 2 O was added in an amount of 2 mmol, 0.759 g. The solution was stirred for 1 h at room temperature, then 80 mL of deionized water was added to the reaction mixture and stirred overnight. The light pink precipitate, which was obtained, was filtrated and dried in a desiccator overnight (yield: 1.194 g, 71%), Anal. Calcd. for C 30  X-ray diffraction were obtained within a week. These were collected by filtration and dried with air.
The IR spectra of complexes 2-6 display general characteristic features. The strong vibrational band observed over the frequency range 1605-1615 cm −1 is typically assigned to the coordinated carbonyl stretching frequency, ν(C=O) [47][48][49]. The broad band centered at 3462 cm −1 in 1a and 3448 cm −1 in 1b is assigned to the ν(O-H) stretching frequency of the coordinated aqua/methanol ligands.
In order to analyze the degree of distortion of the coordination polyhedra for compounds 2-6 from their ideal polyhedron geometry, calculations using the continuous shape measures theory with the SHAPE software were performed [73,74]    Various non-covalent interactions (ring‧‧‧ring, C-H(X)‧‧‧ring [72], hydrogen bonds) are summarized in Tables S1-S5 for compounds 2-6, respectively.
In order to analyze the degree of distortion of the coordination polyhedra for compounds 2-6 from their ideal polyhedron geometry, calculations using the continuous shape measures theory with the SHAPE software were performed [73,74]   Various non-covalent interactions (ring···ring, C-H(X)···ring [72], hydrogen bonds) are summarized in Tables S1-S5 for compounds 2-6, respectively.
In order to analyze the degree of distortion of the coordination polyhedra for compounds 2-6 from their ideal polyhedron geometry, calculations using the continuous shape measures theory with the SHAPE software were performed [73,74]

Photoluminescence of the Complexes
The luminescence spectra of compounds 2-6 were measured in the solid state at room temperature. The excitation spectra recorded at the emission wavelength (λ em ) of 661 nm reveal a broad, intense band around 367 nm for 2-4 and 380 nm for 5 and 6. This broad band corresponds to the π → π* transition from the ligands. The luminescence emission spectra of the samples were recorded upon the excitation wavelengths (λ ex ) of 367 nm for 2-4 and 380 nm for 5 and 6. All spectra display a characteristic band at 661 nm ( 5 F 5 → 5 I 8 ) corresponding to the metal-centered emission and is assigned to the Ho 3+ f-f transition from the 5 F 5 excited state to the 5 I 8 ground state. For this band, the Stark splitting of the degenerate 4f levels under the crystal field is perceived. In addition, compounds 2-4 showed a weak band at 545 nm, which can be assigned to an f-f transition from higherenergy states ( 5 F 4 , 5 S 2 ) to the ground state 5 I 8 [75][76][77]. The triplet states of the ntfa and btfa ligands were calculated by Sato and Wadain Gd(III) complexes [78], taking into account the sensitization effect of the energy transfer from the singlet state of the ligand (S 1 ) to the lowerin-energy ligand triplet state (T 1 ) through the intersystem crossing. These calculations showed that the ntfa T 1 state falls around 19,600 cm −1 for ntfa and 21,400 cm −1 for btfa. Thus, we can suggest that the energy transfer from the T 1 of the ntfa ligand to the 5 F 4 and 5 S 2 (18,348 cm −1 ) thermal state is inefficient because the two states are too close in energy, and as a result, the 5 F 4 , 5 S 2 → 5 I 8 transitions are not identified for compounds 5 and 6, but they are seen for btfa complexes 2-4 [79]. Typical representative UV-Vis and luminescence emission spectra (Vis and NIR regions) are depicted in Figure 6 for 3 and Figure 7 for 6 as representatives of the two categories of 2-4 and 5 and 6 complexes, respectively (for luminescence spectra of 2, 4, and 5, see Figures S12-S14). reveal a broad, intense band around 367 nm for 2-4 and 380 nm for 5 and 6. This broad band corresponds to the π → π* transition from the ligands. The luminescence emission spectra of the samples were recorded upon the excitation wavelengths (λex) of 367 nm for 2-4 and 380 nm for 5 and 6. All spectra display a characteristic band at 661 nm ( 5 F5 → 5 I8) corresponding to the metal-centered emission and is assigned to the Ho 3+ f-f transition from the 5 F5 excited state to the 5 I8 ground state. For this band, the Stark splitting of the degenerate 4f levels under the crystal field is perceived. In addition, compounds 2-4 showed a weak band at 545 nm, which can be assigned to an f-f transition from higherenergy states ( 5 F4, 5 S2) to the ground state 5 I8 [75][76][77]. The triplet states of the ntfa and btfa ligands were calculated by Sato and Wadain Gd(III) complexes [78], taking into account the sensitization effect of the energy transfer from the singlet state of the ligand (S1) to the lower-in-energy ligand triplet state (T1) through the intersystem crossing. These calculations showed that the ntfa T1 state falls around 19,600 cm −1 for ntfa and 21,400 cm −1 for btfa. Thus, we can suggest that the energy transfer from the T1 of the ntfa ligand to the 5 F4 and 5 S2 (18,348 cm −1 ) thermal state is inefficient because the two states are too close in energy, and as a result, the 5 F4, 5 S2 → 5 I8 transitions are not identified for compounds 5 and 6, but they are seen for btfa complexes 2-4 [79]. Typical representative UV-Vis and luminescence emission spectra (Vis and NIR regions) are depicted in Figure 6 for 3 and Figure 7 for 6 as representatives of the two categories of 2-4 and 5 and 6 complexes, respectively (for luminescence spectra of 2, 4, and 5, see Figures S12-S14).  Furthermore, the luminescence emissions of the compounds 2-6 were recorded in the NIR region from 900 to 1600 nm, where three weak bands were detected at 973, 1179, and 1474 nm. The first and most intense band is assigned to the 5 F5 → 5 I7 transition. The band located at 1179 nm accounts for the 5 I6 → 5 I8 transition; the very weak band at 1474 nm corresponds to the 5 F5 → 5 I6 transition [80]. The results obtained here agree with other Ho(III) coordination compounds, where the study of the sensitization of Ho 3+ luminescence by the energy transfer from chromophore ligands has been performed [81][82][83][84][85].

Ac Magnetic Susceptibility Studies
In order to study the dynamic magnetic properties and the possible Single Molecular Magnet (SMM) behavior (slow relaxation of magnetization) of the synthesized compounds, ac magnetic susceptibility, measurements were recorded for solvent-free com- Furthermore, the luminescence emissions of the compounds 2-6 were recorded in the NIR region from 900 to 1600 nm, where three weak bands were detected at 973, 1179, and 1474 nm. The first and most intense band is assigned to the 5 F 5 → 5 I 7 transition. The band located at 1179 nm accounts for the 5 I 6 → 5 I 8 transition; the very weak band at 1474 nm corresponds to the 5 F 5 → 5 I 6 transition [80]. The results obtained here agree with other Ho(III) coordination compounds, where the study of the sensitization of Ho 3+ luminescence by the energy transfer from chromophore ligands has been performed [81][82][83][84][85].

Ac Magnetic Susceptibility Studies
In order to study the dynamic magnetic properties and the possible Single Molecular Magnet (SMM) behavior (slow relaxation of magnetization) of the synthesized compounds, ac magnetic susceptibility, measurements were recorded for solvent-free compounds 2-5. Compounds 2-5 do not show a dependence on the in-phase and out-of-phase components in front of the temperature and frequency, neither in the minimum dc field (0 T) nor in the maximum applied dc magnetic field (0.1 T). Therefore, these compounds do not show slow relaxation of the magnetization and consequently will not show SMM's behavior.

Dc Magnetic Susceptibility Studies
Powder samples of complexes 2-5 were measured under applied magnetic fields of 0.3 T (300-2 K). The data are plotted as χ M T products versus T in Figure 8. Magnetization dependence of the applied field at 2 K for compounds 2-5 was also recorded and is shown in Figure 9.  The magnetic measurement on 2-5 reveals that the χMT values at 300 K are 13.8, 13.7, 13.9, and 14.3 cm 3 K mol −1 , respectively, which are in the range of the theoretical value for a magnetically uncoupled Ho(III) compound (14.07 cm 3 ‧K‧mol −1 ) in the 5 I8 ground state (gJ = 5/4) [86]. On cooling the samples, χMT values remain constant up to 125 K. Below this  The magnetic measurement on 2-5 reveals that the χMT values at 300 K are 13.8, 13.7, 13.9, and 14.3 cm 3 K mol −1 , respectively, which are in the range of the theoretical value for a magnetically uncoupled Ho(III) compound (14.07 cm 3 ‧K‧mol −1 ) in the 5 I8 ground state (gJ = 5/4) [86]. On cooling the samples, χMT values remain constant up to 125 K. Below this The magnetic measurement on 2-5 reveals that the χ M T values at 300 K are 13.8, 13.7, 13.9, and 14.3 cm 3 K mol −1 , respectively, which are in the range of the theoretical value for a magnetically uncoupled Ho(III) compound (14.07 cm 3 ·K·mol −1 ) in the 5 I 8 ground state (g J = 5/4) [86]. On cooling the samples, χ M T values remain constant up to 125 K. Below this temperature, χ M T values decrease to finite values of 6.8, 7.3, 8.9, and 7.4 cm 3 ·K·mol −1 at 2 K for compounds 2-5, respectively. The decrease in χ M T values at low temperatures could be due to the depopulation of the sublevels generated for the spin-orbit coupling and the ligand-field effect (Stark sublevels).
Magnetization dependence on magnetic static applied field at T = 2 K for complexes 2-5 (

(phen)] (2), [Ho(btfa) 3 (bipy)]
(3), [Ho(btfa) 3 (di-t bubipy)] (4), [Ho(ntfa) 3 (Me 2 bipy)] (5), and [Ho(ntfa) 3 (bipy) 2 ] (6) were synthesized from their precursors diaqua tris(β-diketonato) species. The compounds were structurally characterized, where coordination numbers CN = 8 were observed. The distortion of the coordination polyhedra of Ho 3+ centers was analyzed with the SHAPE program. All the complexes display CN 8. In a fashion that is similar to their Ln 3+ analog complexes (Ln = La, Pr, and Nd) derived from the same set of ligands [47][48][49]. The solid-state luminescence emission of the complexes revealed a strong, intense emission band at 661 nm in the visible and three other bands in NIR regions. The magnetic measurements of the complexes 2-5 revealed that the χ M T values are within the range of 14.0 ± 0.3 cm 3 ·mol −1 ·K at 300 K, which is predicted for a magnetically uncoupled Ho 3+ compound (14.07 cm 3 ·mol −1 ·K) in the 5 I 8 ground state (g J = 5/4) [86]. The luminescence emission and magnetic results reported here for the Ho 3+ compounds demonstrate that these properties are not significantly affected by either the small changes in the geometrical shape of the Ho 3+ complexes or their local symmetry. Additionally, results are almost independent of the nature of the ancillary bipyridyl ligands or the nature of the β-diketone coligands. Similar results were obtained with pyridyl adducts derived from the same coligands with Pr(III) and Nd(III) compounds [48,49].